Many reservoirs currently under production suffer from excessive water within the production well. Excess water may enter a production well by a natural water source (e.g. aquifer) where the presence of high permeability zones in the reservoir provides pathways for water to bypass oil-bearing regions and break through into the production wells. Similarly, excess water may enter a production well as a result of production methodologies such as waterflooding the reservoir where water is pumped into a production well to displace any remaining oil in the reservoir matrix after the primary stages of oil production. Areas of the reservoir that are fractured, either naturally or hydraulically, are excellent pathways for floodwater to penetrate and consequently bypass oil bearing pore spaces.
Conformance, with respect to water in oil reservoirs, is the degree to which water sweeps a reservoir uniformly. Conformance problems are created when water does not sweep a reservoir uniformly and lead to excess water production, a reduction in the productive well life, and increase lifting costs inter alia. In general, poor conformance exists in a fractured reservoir due to the very large permeability of fractures compared to the matrix with a result being that an excessive amount of water is produced in the production well. Handling and separating water from produced oil from such wells imposes an economic constraint for oil producers. Furthermore, the presence of fractures impacts the conformance of water and other fluids during injection process negatively. In U.S. Pat. No. 3,762,476 to Gall, it is stated that many oil wells produce a gross effluent containing 80 to 98 percent by volume of water and only 2 to 20 percent by volume oil.
Blocking the highly permeable thief zones thereby diverting water towards the unswept regions of the reservoir has been proposed and used by oil producers as a viable remedy for this problem. In particular, in-depth gel placement is the most widely used technique for blocking highly permeable zones of reservoirs. This technique has been implemented through many field trials around the world and researchers have studied the mechanisms governing this process in porous media.
Previous researchers have proposed processes based on sequential injection of chemical agents to improve the conformance of water in porous media. U.S. Pat. Nos. 3,805,893 and 3,871,452 to Sarem are examples of such processes. In these patents, improved performance is achieved by sequential injection of a chemical, such as dilute alkaline metal silicate solution, followed by another chemical reagent, such as calcium or magnesium. There have since been various improvements to the above process. For instance, U.S. Pat. No. 3,658,131 to Biles describes the use of a slug of fresh water in the above mention process. Similarly, U.S. Pat. No. 2,747,670 to King, et al discloses a similar process based on sequential injection of a brine solution followed by fresh water, followed by a dilute alkali solution. All the above processes are based on injecting inorganic chemicals into reservoirs sequentially. As a result of the chemical reaction, precipitation will occur which would block the highly permeable regions of the reservoir.
In another series of investigations, researchers have tried to improve the Residual Resistance Factor (RRF) for gel systems placed in fractures. In U.S. Pat. No. 4,683,949 to Sydansk, conformance improvement is achieved in a subterranean hydrocarbon-bearing formation using a gel comprised of a high molecular weight water-soluble acrylamide polymer, a chromium III/carboxylate complex capable of crosslinking the polymer, and an aqueous solvent. The gel components are combined at the surface and injected into the desired treatment zone via a wellbore to form a continuous single-phase gel.
In U.S. Pat. No. 3,762,476 to Gall, a method has been provided by injecting a first polymer solution, following this with a crosslinking ionic solution, a brine slug, and then a second polymer solution, after which the injections are terminated. In this method the injected polymer solution is not previously crosslinked.
U.S. Pat. No. 3,981,363 to Gall provides an improvement to the above mentioned method by improving RRF for blocking fractures by injecting into the formation a first partially crosslinked polymer solution, followed by injecting crosslinking agents, and thereafter injecting a second aqueous partially crosslinked polymer solution that is capable of being gelled by the crosslinking agents.
In U.S. Pat. No. 4,039,029 to Gall, a process is provided for reducing the water permeability near wells that have been previously treated with partially gelled or ungelled polymer and from which oil has been produced for a period of time subsequent to the treatment. The retreatment is made by injecting into the formation an ionic solution capable of crosslinking residual ungelled polymer in the formation, and thereafter injecting an aqueous solution of polymer capable of being crosslinked by the ionic solution.
The invention described herein relates to enhancing polymer gel treatments for the purpose of improving conformance and sweep efficiency problems during production and/or injection processes. Although gel placement in fractures is a common practice in the field, continual improvement in the performance of this technique is required. Previous work (Ganguly, S. 2000. Effect of Leak-off on Behavior of Cr (III)-PHPA gel in Fractured Media. PhD dissertation, University of Kansas, Lawrence, Kans.) in the area of gel placement has shown that diffusion of the crosslinker from the gelant placed in the fracture occurs to thin brine films at the fracture face and adjacent matrix. This molecular diffusion of the crosslinker has a significant impact on the performance of the gel within the fracture. All documents referred to herein are incorporated by reference.